If my character is a spell caster, and he is not wearing any armor, is there any chance to fail casting the spell? I'm wondering specifically about normal circumstances: There is nobody who can attack him, and nothing which can interrupt him. Is there a check under normal circumstances to successfully cast the spell — maybe a Spellcraft check?

I ask because it's a little weird to me that a spell caster in normal conditions would have a 100% chance to cast a spell.

3 Answers
3

No, not generally. Spellcasting in D&D or Pathfinder is a routine process -- you select the spell, and then cast it. The character is presumed to be so proficient at this process that there is no chance of failure under normal circumstances.

However, there are some special cases where there is a chance of failure. Two of them require the caster to make a check on a d20 to succeed.

Concentration checks: When something distracts you from casting the spell, you must make a concentration check or lose the spell. Taking damage or choosing to cast defensively are the most common types of distraction, but strong weather, casting from horseback, and several other conditions will also require this type of check.

Spell Resistance: Some creatures have resistance to magic. This can be overcome by making a caster level check. Failure does not cancel the spell, but does prevent the creature from being directly affected.

There are other chances of failures that aren't really "checks", but I thought might be worth mentioning.

Arcane spell failure: Arcane spells which have somatic components can be tricky to cast while wearing armor; there will be a flat % chance of losing the spell. This chance is determined by the type of armor (and shield).

Deafness: A spellcaster who cannot hear has a flat 20% chance of ruining any spell with a verbal component.

Incorporeal creatures: A spell which does damage deals 50% of that damage to an incorporeal creature, but other types of spells simply have a 50% chance of failing to affect them. (Force spells and channel energy have full effect.)

Finally, it is worth including the one additional case where you do have to make a check to cast a spell.

Scrolls above your level: Casting from a scroll whose caster level exceeds your own requires a caster level check. No other type of magic item has this particular quirk; it's a property of scrolls only, not wands or staves.

No, under optimal circumstances there is not a check. However there are a long list of situations that involve a check:

Strong Wind

Situations where you can speak but cannot hear yourself

In grapple

While being stabbed

etc

Note that while the spell may succeed under normal circumstances, spells that negatively affect other creatures may well fail or partially fail due to ranged touch attacks which may miss, or the creature succeeding on a saving throw. Also some monsters have spell resistance which requires a check for your spell to affect them.

Magic in D&D and its derivatives is a very functional, straightforward affair. You learn, memorize and cast, with little chance for failure except under strenuous or unusual circumstances, such as the ones C. Ross outlined above.

This quirk of mechanics is a large part of D&D's magic system's flavor, and directly affects most settings that use the D&D mechanic. It encourages settings where magic is commonplace, since it's a dependable skill to be learned, rather than an arcane and unpredictable art. If it seems weird to you, you might be used to settings where magic isn't a spellcaster's screwdriver, but where magic is always prone to failure, sometimes catastrophically so, like in Ars Magica or other games.